This application is based on German priority application 19930113.1, which is hereby incorporated in its entirety by reference.
The present invention relates and method for filtering a signal, and in particular, to an apparatus and method for filtering a signal which represents a digital data stream.
Systems and methods for filtering signals in information transfer systems are desired, in which the information which is to be transmitted (the digital data stream) is represented by a digital data stream and is modulated onto a high-frequency carrier for transmission. For example, such a system is a system operating on the basis of the DECT standard.
In cases where the digital data stream is modulated onto the high-frequency carrier directly (unfiltered), the resulting signal occupies a very broad frequency spectrum. This is due to the spasmodic nature of the curve of the signal to be modulated onto the carrier, particularly when a plurality of transmission channels with relatively close carrier frequencies are required. For this reason, the data to be transmitted should preferably be subjected to low-pass filtering before it is modulated onto the carrier signal.
Low-pass filtering can be accomplished, for example, using a filter comprising resistors, capacitors and/or inductors. The design of such a filter involves a relatively high degree of complexity, requiring a large number of components. Additionally, the technical data of the components used must not or must only slightly differ from firmly stipulated nominal values.
Another example of filtering a signal (which represents a digital data stream) is to form the signal, by the sum of the output currents or output voltages from a multiplicity of current sources or voltage sources, the current sources or voltage sources being turned on and/or off (for example using the so-called thermometer code) based on the curve of the signal which is to be filtered.
This invention is direct to an apparatus and method for filtering a signal which represents a digital data stream, the signal to be produced being generated using the currents or voltages from current or voltage sources selected from a multiplicity of current or voltage sources.
In one embodiment of the invention, there is provided an apparatus for filtering a signal which represents a digital data stream. The signal to be produced is generated using the currents or voltages from current or voltage sources selected from multiple current or voltage sources.
In another embodiment of the invention, the filtering carried out by the apparatus is performed by filtering a signal which represents a digital data stream. The signal to be produced is generated using the currents or voltages from current or voltage sources selected from a multiplicity of current or voltage sources.
Apparatuses and methods of this type are advantageous because the current sources or voltage sources, in contrast to the elements of the aforementioned filters, can easily be accommodated in integrated circuits, and because tolerances in the technical data for the components used are easier to handle or to offset than tolerances in the components of a conventional filter. On the other hand, the control device driving the current sources or voltage sources (the control device which determines which current source or voltage source is to be turned on and turned off, and when) is relatively complicated. Furthermore, practical implementation in the case of particular integrated circuits (for example in the case of RF modules in which bipolar or BiCMOS technology is used) can take up a relatively large amount of chip surface, which significantly increases the cost of the particular chip.
The present invention is therefore based on an apparatus and method for filtering a signal. The signal represents a digital data stream, and the signal to be produced is generated using the currents or voltages from current or voltage sources selected from multiple current or voltage sources. In this regard, the signals which represent digital data streams can be filtered as desired with a minimum degree of complexity.
In one aspect of the invention, the current or voltage sources whose currents or voltages are used to generate the signal are selected on the basis of the contents of the elements of a shift register (shift register contents) whose input connection has the signal which is to be filtered applied to the input connection.
In another aspect of the invention, the signal which is to be filtered is applied to a shift register, and the current or voltage sources whose currents or voltages are used to produce the signal on the basis of the content of the shift register elements are selected. Only one shift register is required to drive the current or voltage sources, i.e., to define which current or voltage sources need to be turned on and turned off, and when.
Furthermore, the device which drives the current or voltage sources (the shift register) can be accommodated in a very small space (even with integration in any desired integrated circuits).
Signals which represent digital data streams can thus be filtered as desired with a minimum degree of complexity.